This invention relates to a process for the oxidation of hydrocarbons with gases containing molecular oxygen to form the corresonding alcohol-ketone mixtures. More particularly, the invention relates to a process in which the recovery and reuse of the saponification liquor used to purify the reaction mixture is effected by oxidation of the organic by-products dissolved in the saponification liquor.
Hydrocarbons can be oxidised using gases containing molecular oxygen. Oxidation into alcohols is of considerable commercial significance, being carried out in the presence of catalysts or other auxiliary substances in order to increase selectivity.
Typical oxidation catalysts are described for example in DOS No. 2.028.166, and are inorganic and organic compounds of transition metals, preferably cobalt and manganese.
Reference is made here by way of example to cobalt naphthenates, cobalt oleate, cobalt stearate and cobalt ethyl hexanoate and to te cobalt salt of a stoichiometric mixture of the monoester and the diester of isotridecanol with orthophosphoric acid.
U.S. Pat. No. 3,243,449 describes boron compounds as esterification component for the alcohol formed during the oxidation process. The compounds are used in at least stoichiometric quantities, based on the alcohol content of the oxidation mixture.
Typical boron compounds used in oxidation processes of this kind include boric acids, for example orthoboric acid and metabroic acid and boric acid esters such as the monoester of metaboric acid and the alcohol of the hydrocarbon to be oxidised, for example cyclohexyl metaborate when cyclohexane is the starting compound, and boric acid anhydrides such as, for example, B.sub.2 O.sub.3 and B.sub.4 O.sub.5. Mixtures of these boron compounds can also be used.
The reaction mixture is washed with water or hydrolysed with water in order to separate off the water-soluble secondary products formed by the oxidation of the hydrocarbons and, if necessary, the boron compounds used. In industrial processes, the aqueous phase is generally cycled; a component stream of the aqueous solution is removed from the circuit to recover the boron compounds used and to work up the secondary products present therein.
In addition to the required alcohol-ketone mixture, the organic phase left after washing with water contains unreacted hydrocarbon, other monoalcohols and dialcohols, aldehydes and ketones, esters of mono-, di- and hydroxycarboxylic acids and also residues of unesterified carboxylic acids. These mixtures are normally washed with alkalis in order to saponify the esters and to separate off both the residual and the liberated carboxylic acids. The alkaline wash is carried out for example with aqueous sodium hydroxide or a soda solution (DOS No. 2,118,279) in a concentration of from 1.5 to 14%, preferably from 3 to 10% by weight, of sodium ions.
In addition to small quantities of free alkali, the saponification liquor which accumulates during this stage of the process, irrespective of the particular oxidation process applied, contains the alkali salts of the carboxylic acids formed as secondary products together with other products of the oxidation process which, although not acidic, are soluble in aqueous, dilute liquor, such as for example monoalcohols and dialcohols. In order to recover the valuable monoalcohols, the saponification liquor is distilled in the first stage or stripped by blowing in steam. The end liquor left containing in addition to the alkali, large quantities of organic secondary products, is removed from the process. In industrial process, this end liquor generally contains from 5 to 15 % by weight of organically combined carbon or about 30 to 120 kg of C/t of alcohol-ketone mixture produced.
The organic ingredients have to be eliminated in order to protect the environment from pollution. The methods hitherto proposed for this purpose are unsatisfactory. Biological purification involves extremely high costs on account of the large quantities of dissolved organic ingredients. In addition, it is attended by the disadvantage that all the alkali enters the main drainage system with the effluent
Attempts to burn the saponification liquor have revealed significant technical difficulties. The alkaline melt residue which assumulates in the combustion furnace has a pronounced decomposition effect on all lining materials. The same applies to a greater extent as regards residues containing alkali borate which inevitably accumulate during the combustion of hydrolysis liquors from oxidation processes carried out in the presence of boron compounds. This results in inadequate running times for industrial combustion installations.
A process has now been found by means of which alkalis can be recovered and, at the same time, the organic ingredients of the described saponification liquor eliminated. Accordingly, the invention relates to a process for the oxidation of hydrocarbons during which the hydrolysis liquor is purified by wet oxidation of the organic impurities into essentially carbon dioxide and water by treatment with gases containing molecular oxygen at elevated temperatures and under elevated pressure. The alkali carbonate/bicarbonate liquor formed is returned to the process as saponification liquor.